It is definitely a good idea to borrow from the library. I was going to buy a few Christmas books that I can read to my ds every single year. I borrowed quite a few books yesterday, and I know that had I bought some of them I would have been disappointed.
For anyone that is interested:-
Jeaus' Christmas Party by Nicholas Allan - Very disappointed. Have heard good things about it, but I didn't think it had anything going for it.
When it Snows by Richard Collingridge - Beautiful pictures, very short story. Bit of a rubbish storyline.
My Penguin, Osbert by Elizabeth Cody Kimmel - Nice storyline, gorgeous pictures. Quite a few Americanisms. Main character shares ds's name so I think that made me like it more 
Room For a Little One by Martin Waddell - Absolutely stunning illustrations. Simple retelling of the Nativity story. I think it would hold the attention of a young child.
The Little Reindeer by Caroline Repchuk - Ok book. Just average really. Bit of a predictable story, but maybe that wouldn't matter to a small child.
The Mole Family Christmas by Russell Hoban - Lovely little story. About a mole who finds out all about Christmas, and "The fat man in a red suit". Wouldn't hold the attention of a young child, but is lovely for when slightly older. My favourite 
Of course, everybody likes different things. I thought I would just post about them in case other people were still looking for books to read in the lead up to Christmas.